The UV coatings, UV print inks and systems which produce graphic transference under exposure after being developed, such as resin plate, photogravure printing plate and dry film photoresist, liquid photoresist and photo-image type liquid solder resist serving as a permanent protecting film, are generally cured by free radical photo-polymerization; the photoinitiator is irradiated with ultraviolet rays to yield free radicals which then initiate the chain addition-polymerization of unsaturated polyfunctional photoreactive materials to produce a network structure.
Such free radical polymerization systems are of great importance in industrial practices and the formulations have been continuously improved. However, due to the inhibition of free radical curing by atmospheric oxygen, the photosensitivity and resolution are often unsatisfactory.
Taking UV ink (coating) as an example, after the screen printing, the coated substrate (usually a plate) is passed through a conveyor into a UV drier. During the process of UV irradiation, oxygen in the air attacks the surface of the ink and thus greatly reduces the degree of crosslinking. Hence, the film may be etched by acids, bases or organic solvents such as methyl ethyl ketone, trichloroethane when subject to the subsequent cleaning procedure. Another example is liquid photoresist or liquid solder resist, to which volatile solvent (boiling point less than 250.degree. C.) is usually added to facilitate the coating operation such as dip coating, roller coating, curtain coating or screen printing. In the selective exposure process, the solvent should be removed by preheating so that the photoresist or solder resist is dried and free from tackiness, and then the film having graphs (artwork) together with a photoresist or solder resist will be exposed. Because the substrate or the surface of the photoresist layer is not smooth enough, some air remains between the film and photoresist, and oxygen inhibition results in insufficient crosslinking. In this system, however, materials containing carboxylic groups such as water base photoresist or solder resist are washed with developer at the subsequent stage, and thus not only the unexposed portion is washed away, but also, to a certain extent, the exposed portion which was inhibited by oxygen. As to solder resist, both nonhomogeneity of appearance and tin contamination during soldering result from inhibition of oxygen. As to dried film-resist, due to light scattering caused by the protective membrane between the film and photoresist layer which prevent oxygen inhibition, the resolution is reduced.
The reduction of oxygen inhibition is discussed and disclosed extensively in the literature, for instance, "Photopolymerization of surface coating" by C. G. Roffey, chapter 3, p.127 to 130, (John-Wiley & Son Ltd. 1982) discloses several methods of achieving a reduction of oxygen inhibition including application of a barrier layer, using more powerful-intensity lamps and combining benzophenone/tertiary-amine, etc.
Other patent literature also makes disclosures:
U.S. Pat. No. 3,753,720 discloses that the polythiolpolyene system can achieve sufficient photo-curing effect, however, the system has a severe odor problem.
U.S. Pat. No. 3,759,807 discloses that using a coinitiator such as benzophenone/triethanolamine may increase the curing speed, however, the odor problem still exists. When the co-initiator is applied to the acid contained resin systems, precipitation may occur.
EP 0,037,604 and EP 0,037,152 disclose that by using initiator systems containing alkylsulfonate and sulfonic groups may accelerate the surface curing speed. However, use of the ionic compounds may effect the electrical properties and the sulfonic group containing initiator is difficult to synthesize. The possibility of commercial use is very low.
Another literature concerning oxygen exhibition is disclosed in "Materials for Microlithography", by A. Hult and B. Ranby, p.457, ACS symposium series 266, which makes use of the following initiator: ##STR1## to accelerate surface curing speed. However, from an economic point of view, it is unlikely to be put into practical use.
Therefore, reduction of oxygen inhibition practical for use in industrial production is an urgent need. In the present invention, surprisingly we find by applying the high boiling point compounds in the photoreactive composition which comprise the following formula: ##STR2## wherein
R.sub.1 is hydrogen liner alkyl or acyl groups having 1-20 carbon atoms;
R.sub.2 is hydrogen or methyl group;
in a photopolymerization composition. The photosensitivity is obviously increased and adding the photosensitive promoter into the composition will not destroy or reduce the originally designed performance.
Generally, the present composition is a photocurable mixture which comprises at least
(a) a material having unsaturated carbon-carbon double bonds which undergoes free radical polymerization;
(b) a photoinitiator which produces free radicals when irradiated with ultraviolet light; and
(c) a photosensitivity accelerator of the formula ##STR3## wherein
R.sub.1 is hydrogen, linear alkyl or acyl groups having 1 to 20 carbon atoms; n=4-20;
R.sub.2 is hydrogen or methyl group; and the complete formula
has a boiling point greater than 300.degree. C. The composition, when mixed with other conventional additives, obtains liquid photoresist, UV-ink or coating or photo-image type liquid solder resist with higher photosensitivity.